Multiplexing holograms means to store multiple holograms in the same volume or nearly the same volume. Typically, this is done by varying an angle, wavelength, phase code, or some other system parameter in the recording and readout setup. Many of these methods rely on a holographic phenomena known as the Bragg effect in order to separate the holograms even though they are physically located within the same volume of media. Other multiplexing methods such as shift and, to some extent, correlation use the Bragg effect and relative motion of the media and input laser beams to overlap multiple holograms in the same volume of the media.
Some multiplexing techniques use momentum (spatial frequency) to filter out the unwanted reconstructions from the desired reconstruction. Examples of these methods include: fractal, aperture (disclosed, for example, in U.S. Pat. No. 5,892,601 to Curtis et al for “Multiplex Holography, which is incorporated in its entirety by reference) and peristrophic multiplexing. Each of which is understood in the art. For disclosure of aperture multiplexing see U.S. Pat. No. 5,892,601 which is incorporated in its entirety by reference, and for a disclosure of peristrophic multiplexing see K. Curtis et al “Method of Holographic Storage Using Peristrophic Multiplexing”, Optics Letters, Vol. 19, No. 13 pp. 993–994, 1994 and U.S. Pat. No. 5,483,365 to Pu et al. for “Method for holographic Storage using Persitrophic Multiplexing” each of which is incorporated in its entirety by reference. By changing the reference beam angle and moving the media between recordings, the reconstructions are still Bragg matched but come out at different angles and can therefore be filtered out.
Using holography to store data has been well known for the last 30 years. The idea of increasing system capacity by combining spatial multiplexing (recording holograms in multiple locations but not significantly in the same volume of media) along with some other multiplexing technique that overlaps holograms within the same location has been well known for over 15 years. These are standard techniques for distributing holograms on holographic media such as a disk, card, cube, or tape. Several patents and papers disclose a number of multiplexing techniques: U.S. Pat. No. 5,550,779, “Holographic Memory with Angle, Spatial and Out of Plane Multiplexing”, and S. Li, “Photorefractive 3-D Disks for Optical Storage and Artificial Neural Networks” California Institute of Technology, pp. 78–111, 1994, each of which is hereby incorporated by reference. All of these place the beam waist, that is, the point at which the beam is focused and the beam spot size is smallest, (either image or Fourier transform plane) inside the media. By doing so, relatively small holograms can be generated which make excellent use of the media material's dynamic range.
FIG. 1 illustrates a prior art method of spatial and angle multiplexing holograms in a relatively thick media. FIG. 1 shows a holographic media 8 in which an angle multiplexed hologram is being created by reference beam 20a and signal beam 10a. In FIG. 1, signal beam 10a includes an incoming converging cone 12, an outgoing diverging cone 14 and a waist 16, where the signal beam is focused in the media 8 and where its spot size is smallest. FIG. 1 also shows reference beam 20b, which can be used to generated a second hologram in media 8 that is angle multiplexed with the hologram generated by reference beam 20a and signal beam 10a. A number of holograms, or stack, can be angle multiplexed in a portion 24 of the media. The media or signal source can the be shifted to record a second stack of holograms. FIG. 1 illustrates signal beams 10b, 10c and 10d which, along with reference beams shown in phantom, generate additional stacks of holograms in portions 24b, 24c, and 24d, respectively, of media 8. In FIG. 1, the portions 24a–24d of media 8 outline the area used by each stack.
Portions 24a–24d are significantly larger than an individual beam waist, such as beam waist 16. This is because both the signal beam and the reference beams determine the area that a given hologram stack uses. To spatially multiplex these holograms, stacks of holograms must be separated by at least the length of a portion 24a–24d of media 8. This has consequences for achievable densities and capacities that can be reached using holographic storage. High density is achieved by multiplexing more holograms in one location and by placing these stacks as close as possible. However, as discussed above, close spacing of stacks is limited.
Additionally, the divergence of a beam can limit the minimum distance between stacks. The amount of divergence, which can be expressed as the angle the edges of the diverging cone form with the direction of beam propagation, is dependent on the numerical aperture of a lens through which the signal beam is projected. For high NA systems that are typically used for storage systems, the amount of signal beam divergence in holographic media, such as media 8, is relatively significant for relatively thick media. In addition, the number of holograms that can be multiplexed at one place (one stack) is determined by the thickness of the media. More holograms can be stored in thicker media due to the increase in the Bragg selectivity and dynamic range. Unfortunately, if the media is made thicker the spatial stack size increases due to the increased divergence of the beam. Thus the achievable density/capacity saturates at a certain thickness. Thus, density cannot be increased significantly by increasing the material thickness once the saturation thickness is reached.
Increasing density is also possible by overlapping holograms. An example of this with angle multiplexing is disclosed in “Spatioangular Multiplexed Storage of 750 Holograms in an FeLiNbO3 Crystal”, Optics Letters, Vol. 18, No. 11 pp. 912–914, 1993, which is incorporated in its entirety by reference. With this concept, partially overlapping hologram stacks are recorded with angle multiplexing within each stack. Each stack, however, has a unique set of angles and therefore, though the stacks partially overlap, the holograms can be easily separated. This increases the density of the stacks but many fewer holograms can be recorded in a stack, which very significantly reduces the density gains of overlapping the stacks. In practice this method results is very little if any increase in achievable density. When multiplexing holograms, however, the dynamic range of the holographic media can be a limiting factor. (The materials dynamic range or M# is a measure how many holograms can be multiplexed at a given location in the material and is related to the materials index change and material thickness.) Thus, the reduced possible number of angle multiplexed holograms was acceptable since it reduced the demands on the available dynamic range for a given overall density. This is because as more holograms are multiplexed in the same volume (i.e. angle multiplexed) the diffraction efficiency of the holograms drops depending the material dynamic range (M#) divided by the number multiplexed holograms squared. Now that better materials have been invented, a way of actually increasing the achievable density is needed.